The elderly suffer an increased prevalence of morbid disorders of sodium and water balance. This is secondary to many age and disease related alterations in the underlying physiologic homeostatic mechanisms regulating extracellular fluid volume and composition. To examine the influence of increased extracellular volume on physical functioning and hormonal regulatory indices, we are studying healthy young and old volunteers ranging in age from 18 to 30 years and 70 years and older. After eating a controlled normal sodium diet for three days, each volunteer undergoes 24 hours of four-hourly blood sampling for atrial natriuretic peptide level (ANP), two consecutive 12-hour urine collections, and an exercise test, during which ANP, vasopressin, norepinephrine levels and non-invasive estimation of pulmonary capillary wedge pressures are obtained. The study is repeated approximately a month later after the volunteer has consumed a 250 mEq (high) sodium diet for three days. Data analysis will elucidate the relationship among the endocrine, cardiovascular and renal systems of young and old sedentary volunteers when salt intake is varied from normal to high. Diurnal variation of ANP secretion and sodium excretion between age groups will also be examined.